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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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GLACIATION OF THE YAKUTAT BAY REGION                203
of coarser fragments where they were brought by the glacier, and especially in marginal deposits. The glacial streams issue with great volume in this climate, and they bear vast quantities of sediment, varying from clay to coarse gravel and small bowlders. 'Where the grades of the valleys are steep enough, much of this debris is borne to the fiord, but, in those valleys with flatter grade, outwash gravels accumulate; though even in these cases extensive deltas are built in the fiord, and vast quantities of clay are contributed to the deposits being laid down on the fiord bottom. At times, as in the case of Hidden Glacier, outwash gravel plain and delta coalesce, and from the glacier front to the sea there is a continuous outwash gravel plain, growing finer in texture toward the sea, and beyond mid-tide level consisting mainly of clay. Such deposits, extending from one side of the valley to the other, overspread and bury from view those deposits that were previously made beneath the glacier, or by its melting; and, in fact, they sometimes bury the glacier end itself, giving rise to extensive buried ice blocks.
Some of the smaller glaciers that terminate in the position here being considered have their ends covered by ablation moraine and are in a stagnant state; and in some cases the lower ends are even disconnected from the upper part of the glaciers. Under these conditions the rate of ablation is decreased, the extent of outwash gravel deposit is diminished, and neither glacier ends nor glacier deposits are extensively buried beneath the gravels. In such cases the record of former presence of glaciers will include a sheet of slightly-irregular moraine, including a large percentage of angular, frost-riven blocks. One might expect to find eskers in such situations, but not where the recession of glaciers is so closely followed by gravel deposit as to cover both previous deposits and glacier ends. In the latter case, however, conditions are favorable for extensive development of kame topography when the buried ice blocks melt and the overlying gravels slump irregularly.
The Nunatak Glacier is the only case in this region of a tidal glacier which terminates in a mountain valley which it completely occupies from side to side. Ablation is rapid and iceberg discharge active, the two causes together sufficing to cause rapid recession of the glacier front. Marginal drainage is not developed, but subglacial or englacial streams pour volumes of water into the fiord in front of the glacier. Doubtless these are giving rise to extensive sedimentation in the neighboring fiord, but owing to the rapid recession of the glacier front these deposits are spread over a wide area. Should the front halt for a time the vast quantities of sediment that are doubtless pouring out from the ice front would build notable terminal deposits. In the deposits that are accumulating both fine and coarse material are of necessity included, thus differing notably from the outwash plain deposits in which the clay element is almost completely absent. These submarine deposits must also differ from those of the land streams in the presence of large rock fragments dropped from the glacier front or from the icebergs; but the number of these fragments is not great, for the larger proportion of the material that the ice bears to its front is carried away in the icebergs. This deposit should be assorted throughout, for in addition to the fact of its deposit in water, in which there are strong tidal currents, it is brought to its place of accumulation by rapid streams, whose continued progress is arrested by the mass of standing fiord water.
The other two tidal glaciersTurner and Hubbarddiffer from the Nunatak in one very notable respect. They extend beyond enclosing mountain walls and spread near their terminus. Were the fiord water absent, these glaciers would doubtless develop